Conventionally, as methods for producing a 3-halo-1,2-benzisothiazole,
(A) a method including reacting 1,2-benzisothiazole and phosphorus oxychloride or phosphorus trichloride in the absence of a solvent, to give 3-chloro-1,2-benzisothiazole (see, Patent Publication 1):

(B) a method including reacting 1,2-benzisothiazole and phosgene in the presence of a catalyst such as dibutylformamide, to give 3-chloro-1,2-benzisothiazole (see, Patent Publication 2):

(C) a method including reacting thionyl chloride and methylamine with dithiosalicylic acid to produce an amide form, and subsequently reacting the amide form with phosphorus tetrachloride, to give 3-chloro-1,2-benzisothiazole (see, Patent Publication 3):

and the like have been known.
However, the method (A) has some disadvantages that separation of a phosphorus halide which is a side reaction product is difficult, thereby necessitating the use of a phosphorus oxychloride or a phosphorus trichloride which has some disadvantages in wastewater treatment, and that the yield is also low. The method (B) has some disadvantages that phosgene which is very highly toxic and has some disadvantages in handling safely, so that the method can hardly be said to be an industrially advantageous method. The method (C) has some disadvantages that expensive dithiosalicylic acid is used, that a large number of reaction steps need to be taken, and that the yield is also low, so that the method can hardly be said to be an industrially advantageous method.